In the FPD (flat panel display) industry, a large number of testing equipment use glass stage as a platform for carrying glass substrates, such as CD (critical dimension) measuring equipment, total pitch (total spacing) measuring equipment, maintenance equipment, etc. They are designed to utilize a translucent property of the glass stage to provide a translucent illumination from location below the stage during measurement, so as to increase the contrast and clarity of the measured image and then get more accurate and stable measurement results.
Specifically, during a measurement a light source for translucent illumination is usually placed under the glass stage. After passing through the glass stage and the glass substrate, the light of the light source enters a measurement microscope and generates a measured image at a CCD (Charge-coupled Device).
However, this design is also flawed. In order to firmly fix the glass substrate during a measurement and to facilitate positioning of the substrate during transporting the substrate, a large number of vacuum suction and air blowing lines need to be provided on the glass stage. Joints of these lines are usually opaque resin or metal material. When the light source is moved to a position below the glass stage to irradiate the glass substrate, the light of the light source will not be able to pass through the glass stage due to the obstruction of the joint, thus forming a “blind spot” for lighting, and affecting the measurement.